Projectors, such as those used to project images in a conference room, to project data in an aircraft's head-up display (HUD), or to project a movie image in a theater, often rely upon a light source behind an image medium to provide the light that is projected to form the image. The light passes through the image medium, such as a layer of film or a liquid crystal display panel, or perhaps is produced by illuminating phosphors on a cathode ray tube, to produce a light image that is projected through a series of lenses toward a target.
Design and construction of the lens assemblies plays a substantial role in the resulting quality of the projected image. Antireflective coatings and optical coupling are used to reduce reflections between lens elements. Specialized glass types are used to reduce chromatic aberration in which different colors of light take different paths through the lens. Careful shaping of the lens elements to correctly redirect problematic off-center light rays that might produce an out-of-focus image with traditional round or flat lens surfaces is also employed. Each of these techniques are commonly employed to ensure that an image having the desired degree of focus, contrast, and brightness can be produced.
But, these techniques are technically complex to employ, and can require sophisticated and expensive production equipment and materials. A user of even relatively expensive binoculars or camera lenses may well notice that images are less sharp near the edges of an image, and that different colors begin to form separate images near the image edges.
It is also desirable in many applications to reduce the quantity of light that proceeds through a lens group but does not accurately contribute to the desired viewing area in the observation plane. Sometimes various other techniques can be employed in a lens system to reduce unwanted light from proceeding through the lens elements, such as where vignetting reduces the number of light rays produced from off-center light sources that proceed through a series of lens elements. This is often undesirable for display applications, though, as the image formed by a solid image such as a frame of film, a CRT image, or an LCD shutter comprises useful light rays emitted from off-center portions of the image. Shuttering or blocking undesired light rays near the observation plane can reduce the demands placed upon the lens elements in handling off-axis light rays that are considered undesirable, but is impractical for many of the previously discussed applications.
It is desired to have a lens system in which the performance requirements on lenses can be relaxed while still maintaining acceptable image quality.